I doubt it. The Eeveelutions follow a specific typing pattern; their types are always something in the card game. The only eligible types left are fighting and dragon. While Dragon seems the most likely since it's Kalos, and you people know Kalos, I highly doubt that they'll add it because Sylveon was the last Eeveelution to be released, and the only one after the original three to not come in a pair, meaning that Sylveon very well could have been the last Eeveelution to be released. While Dragon could pair with Sylveon, since it's Kalos, there's probably a reason why Sylveon was released on its own.